The Burger Lab: The Ultimate Homemade Sliders

It's time for another round of The Burger Lab. Got a suggestion for an upcoming topic? Email Kenji here, and he'll do his best to answer your queries in a future post. Become a fan of The Food Lab on Facebook for play-by-plays on future kitchen tests and recipe experiments.

The Ultimate slider. [Photographs: J. Kenji Lopez-Alt]

By this point, any A Hamburger Today reader knows that a slider is not just a mini burger. Putting a small beef patty on a tiny bun does not make it a slider, and don't even think about applying the moniker to a mini crab-cake or grilled chicken breast sandwich without copious use of quotation marks.

Invented in Wichita, Kansas, in 1916 by Walter Anderson (who five years later founded White Castle), sliders were at one time the predominant form of burger on the planet. Weighing in at under two ounces, the diminutive sandwiches are made by slowly steam-griddling thin, all-beef patties on a bed of onions.

The aromatic steam from the onions wafts through and around the beef and buns, which are placed directly on top of the patty as it cooks. When fully steamed through, the buns become mere wisps of moist pillowy bread—the physical manifestation of sweet, pungent onion vapor. Topped with melty American cheese and a couple slices of pickle, it's the cheeseburger in one of its purest, most noble forms and as a genre, is completely unimprovable.

That said, there are certainly good sliders and bad sliders our there, and the basic methodology leaves plenty of room for interpretation Should the onions be sliced or diced? How cooked should they be? What order should the burger be stacked in? How does the meat get cooked? Is it even possible to dissect and reassemble the slider in standard Burger Lab style and come out the other end with a recipe that doesn't in some way dishonor a nearly 100-year-old institution?

My goal this week: develop the ultimate slider recipe. And as I quickly discovered, there's a lot more going on between those tiny buns than meats the eye*.

* I know, ouch. Sorry about the terrible pun

Heading to the Source

Living a stone's throw from North Jersey, I fortunately have some good stomping grounds to research sliders that are still cooked via the method pioneered by White Castle. Almost all of the slider joints in Jersey have the word "white" in their name; an epithet originally intended to evoke a sense of purity and cleanliness to calm a beef-wary 1920s public, who were still reeling at the horror stories of the Chicago stockyards chronicled in Upton Sinclair's The Jungle.

So how do they cook their burgers? To quote slider expert Nick Solares, this is how it's done (I'll refer to this method as the "traditional" method from here on out):

The method of preparation, despite some variance in ingredients (the way the onions are apportioned, for example) is constant: Small two-ounce (or smaller) cubes of beef are mashed on to a flat top griddle, and onions (either finely chopped into tiny cubes or thinly sliced into delicate ribbons) are pressed into the top of the sizzling patty, which ends up on the bottom when the burger is flipped. If cheese is ordered, it is applied as soon as the patty is flipped, and the bun—top half first, bottom portion placed on top—covers the onion-beef-cheese stack.

After steaming through, the sandwiches are closed, and pickles and condiments are added. Simple, right?

White Castle on the other hand, has made a few innovations in the process over the years that bear some investigation.

Rather than starting with the beef, they first cover the griddle with a layer of reconstituted dehydrated onions (apparently they made the switch from fresh onions during World War II food shortages and have never looked back). The onions and their liquid are allowed to slowly warm and soften, before frozen patties of beef with the distinct five-hole pattern (intended to hasten cooking time) are added, and topped immediately with both bottom and top bun.

So what does this mean? Well, the key difference is that at White Castle, the beef never touches the griddle. The cooking is done 100% via steam from the onions.

It's unclear exactly why White Castle made this change (which according to my research, happened very early on, well before the 1950's). It could be for efficiency and uniformity reasons, or it may be that the method simply delivers better flavor. Clearly, the independent stores in Jersey trump White Castle in terms of quality and freshness of ingredients—but has their cooking method been left wallowing in a less-than-ideally-flavored past?

Armed with beef, onions, and a boatload of questions, I hit the kitchen to find out.

The Bun

First things first. Finding a slider-sized bun is not easy. While most old fashioned slider joints use white, Wonder Bread-esque squishy buns (according to burger expert Josh Ozersky, these are the only buns a slider should be made on), these buns are not readily available to the general public.

While the tiny, 2-inch Party Rolls may be good for passed hors d'oeuvres at a fancy dinner party, I decided to go with the slightly larger Sliced Rolls, which are still small enough to be considered a snack, but large enough that you can make a meal our of three or four of 'em.

The Beef

Another easy decision. Since their thinness prohibits cooking these burgers to anything less than well done to very well done, I knew I'd have to use a very fatty cut. Either fresh ground chuck or fresh ground second-cut brisket would do me just fine. After some brief experimenting with beef-to-bun ratios, I settled on 1.5-ounce patties flattened into three and a half-inch disks to just barely cover the surface of the bun.

The Onions

Here's where we get to the really tough part. In my estimation, the onions are in fact a more important element to the slider than the beef itself. They add moisture, they add flavor, they provide that all-important aromatic steam. Without onions, there can simply be no slider.

My first test was to cook four patties side by side, using cooking method #1, with the onions smashed into the uncooked side of the patty before being flipped. I tried onions cut four different ways: small dice, really small dice, finely sliced parallel to the equator, and finely sliced pole-to-pole.

Straight away, the onion cut parallel to the equator was out for its unappealingly stringy texture. Of the remaining options, the teeny tiny diced onion and the onion sliced very finely pole-to-pole were in a dead heat. They both seemed to have much more distinct onion flavor than either of the other two batches. What was going on?

To get at the answer, we've got to take a quick look at an onion under a microscope:

Onions, like all vegetables, are composed of a series of liquid-filled cells. Within this liquid floats a number of large compounds armed with sulfurous molecules that are designed specifically to irritate our eyes and noses. Now normally, these molecules are inert. Like a bomb without a trigger, they are perfectly harmless. However, as soon as an onion is cut open, its cells rupture and the compounds spill out and come into contact with enzyme triggers that were stored within different compartments.

The enzymes cause a chain of chemical reactions that develop the familiar distinctive pungent odor that, despite their initial role as a defensive deterrent, us humans have come to love.

So this explains why the onions sliced pole-to-pole and finely diced developed more flavor than their counterparts: both methods end up rupturing more cells. I decided to go with the fine dice, for purely textural reasons.

My next thought seemed like a logical one: Could I somehow play up these onion flavors even more?

Salting the Wound

As anyone whose ever made cole slaw, kimchi, or eggplant parm knows, salting vegetables can have a profound impact. When salt first hits the surface of the vegetable, it begins to dissolve in the small amount of liquid present, creating a super-concentrated brine. This brine creates osmotic pressure on the cell walls, causing liquid from the interior to be leached out, along with some of its dissolved solids.

I set up a quick experiment by placing two bowls of chopped onions side by side. One was left plain, while the second was seasoned with kosher salt and allowed to rest for ten minutes.

A quick sniff test confirmed it. The salted onion was considerably more pungent than the unsalted.

Try it out for yourself—the difference is truly remarkable. What's more, the flavor differences were detectable even after having been cooked into the patties*.

*Yet another piece of evidence that seasoning your food while cooking is much more effective than adding salt at the table.

The Cooking Method

Surprise victor here. After extensively testing both the traditional method and the modern White Castle method, I came to the conclusion that given an equal set of starting ingredients, the White Castle method is in fact superior. Allow me to explain in three major points:

Beef Flavor: While in most burgers a crisp, well-seared crust is desirable, a slider is different. The griddle is not kept hot enough to impart significant browning. If it were, it'd rapidly burn the onions to a crisp. Thus the flavor advantage ostensibly offered with the traditional method by cooking the beef in direct griddle contact offers no significant benefit. Cooking on a bed of onions, however, imparts significantly more flavor to the meat. Score one for the White Castle system.

Beef Texture: Cooking onions until fully softened and lightly caramelized (the ideal stage for a slider) is a slow process that takes anywhere between five and ten minutes during which time the beef is continuously cooking. With the traditional method, it gets a jump start on the griddle, and inevitably ends up well beyond well-done. Now, the soft onions and gooey cheese help mitigate its dryness to a degree, but not nearly as much as keeping the beef away from direct contact with the griddle. Score two for the White Castle system.

Onions: Properly softening the onions is tricky business. With the traditional method, the onions are completely obscured by the burgers as they brown. Gauging their doneness is largely a matter of experience cooking onions on the same griddle every single day—something which I'm not prepared to do, and my wife flatly refuses to (along with burgers, wives have also apparently changed since the 1920s). With the White Castle system, you've got an advantage: simply cook the onions sans-burger until they reach the perfect degree of doneness, then add a bit of water, halting them from overbrowning, softening them every so slightly, and creating ample steam for your buns to soak up. Score three for the White Castle System.

The Steam

Not much new here. My only innovation was to stagger the top buns so that they are not directly stacked on top of the bottom buns. This offers better steam-absorption, and prevents even the tiniest wisp of flavor from escaping from around the edges of the pan.

Getting Juiced

At this point, I was already pretty darn happy with my results. By combining the fresh ingredients of a traditional slider with the improved cooking method offered by White Castle, I'd eaten several of the best sliders I've ever had. But there was still one little point that was irking me: adding water to the skillet to halt the onions from overcooking.

Years of restaurant training has taught me that whenever I'm about to add water to something, my first instinct should be to question whether there is another, more flavorful option. Because you see, despite what bottled water marketers will have you believe, plain water is pretty bland.

So what's the liquid of choice?** Why, onion juice, of course

**Yes, yes, in most other cases, Scotch

I pulled out the juicer that I got my wife for her last birthday and after very briefly considering whether the onion smell might potentially soak into the plastic, I decided that in the interest of pure science, the difficult sacrifice of my wife's normally sweet breath was an unfortunate, but necessary piece of collateral damage.

I went ahead and juiced the onion. Asides from having accidentally created a monster of a machine that permanently delivers a morning's worth onion breath with every glass of freshly squeezed mango juice, I had in my hands a potential cup of pure liquid slider flavor. White Manna, indeed!

I cooked up another batch of salted diced onions in a skillet over low heat until very lightly browned, then poured in some onion juice and stirred up and redistributed the contents before adding my patties. I topped each with a slice of cheese, then placed the buns on top to steam, adding a touch more onion juice whenever the pan seemed in danger of drying out.

A few minutes later, I assembled the sandwiches (carefully spooning on any extra onions and melted cheese that had collected in the skillet), topped each with a couple slices of pickle, and dug in.

They were truly phenomenal. Deep aromatic onion flavor, with beef that, despite its size, was moist, juicy, and had even retained a bare hint of pink.

Don't have a juicer? Not to worry. Grating an extra onion on the medium holes of a box grater provides ample juicy pulp to moisten your pan with, providing results that are nearly as good.

With my testing done and everything finally in order, I leave you with two questions:

1. On a slider, what is the ideal stacking order for beef, onions, and cheese? My inclination is that the cheese should be at the bottom, so that it is the first thing your tongue hits as you bite through, thereby coating everything else in a gooey shroud. The beef should be next, and the onions should be on top in order to continue to bathe the burger in their juices. Your thoughts?

2. Given that my wife comes home to a kitchen full of dozens of burgers with one bite taken out of each on a regular basis, how long before she leaves me?

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